Pass on to the next middleware in case of express

As I explained [here](octokit#509 (comment)), it is now required to always install this middleware to the end of the chain, otherwise it rejects all requests it cannot handle.

Author: honnix

src/middleware/node/middleware.ts

@@ -23,8 +23,12 @@ export async function middleware(
 
   const isUnknownRoute = request.method !== "POST" || pathname !== options.path;
   const isExpressMiddleware = typeof next === "function";
-  if (!isExpressMiddleware && isUnknownRoute) {
-    return options.onUnhandledRequest(request, response);
+  if (isUnknownRoute) {
+    if (isExpressMiddleware) {
+      return next();
+    } else {
+      return options.onUnhandledRequest(request, response);
+    }
   }
 
   const missingHeaders = getMissingHeaders(request).join(", ");